1. Field of the Invention
This invention relates to an image forming apparatus employing toner to form a toner image on a sheet. More particularly, this invention relates to the fixing means of the image forming apparatus, which includes rollers for pinching and heating a sheet carrying a toner image thereon so as to fix the toner image on the sheet.
2. Description of the Related Art
In an image forming apparatus, a sheet carrying a toner image formed onto it by the well-known technique passes thought a fixing unit, which comprises a pair of rollers, of which at least one is a heat roller, to have the toner particles melted and bonded onto the sheet. The other roller in the fixing unit is usually a backup roller, such as those in the embodiments disclosed in the following U.S. patents: U.S. Pat. No. 4,949,130 to Torino; U.S. Pat. No. 4,949,131 to Ito; and U.S. Pat. No. 4,965,640 to Watarai et al. An alternative design includes two heat rollers, such as the embodiment described in U.S. Pat. No. 4,905,051 to Satoh et al. The rollers are pressed transversely against each other so that the sheet is pressed tightly and evenly against the heat roller to have the toner particles melted and bonded on the sheet. It is required that the temperature of the heat roller should be kept in a predetermined temperature range. If the temperature is too low, the toner particles may not bond strongly enough on the sheet after the sheet is processed by the fixing unit. If the temperature is too high, the over-melted toner may smear on the sheet, therefore lowering the resolution of the image.
In conventional image forming apparatus, the heat roller is heated by a heat generating means inside the hollow roller. The heat generating means is usually a heat lamp, which is employed in the embodiments described in all the above mentioned U.S. patents. The temperature of the heat roller is usually controlled by bimetallic thermoswitches, such as in a Digital LN-03 laser printer, or an Apple Personal LaserWriter LS printer; or by an electronic circuitry with a thermistor sensor, such as the designs disclosed in U.S. Pat. No. 4,905,051 to Satoh et al., and in U.S. Pat. No. 4,949,131 to Ito.
There are several shortcomings for the conventional means of heating the heat roller and controlling the temperature. First, the heat roller is heated indirectly by the heating means inside, but not in contact with, the roller. Because the thermal inertia is rather large and the heat is transferred mainly by radiation which is not very efficient, it takes some time to raise the temperature of the outer surface of the heat roller to a predetermined temperature, which typically ranges from 160.degree. C. to 190.degree. C. Therefore the fixing unit of an image forming apparatus could not become operational immediately after the power is turned on. For this reason, the heat roller is usually kept hot even during the time period when the apparatus does not process any sheets. A timer circuitry is employed in some of the apparatuses that turns off the heating circuit after a period of inactivity to reduce the waste of energy. However the user will have to activate the heating circuit and wait for another heat-up period to have any sheets processed by the apparatus. Second, when the temperature is controlled by a bimetallic thermoswitch, overheating and underheating can occur during the on/off cycles of the circuit. The fluctuation of the temperature affects the quality of the fixed toner images. Third, the useful life of such bimetallic thermoswitch is limited because of the wear of the contacts. Replacing such thermoswitch usually requires professional service, making the maintenance of the image forming apparatus very expensive. Fourth, the wear of the contacts of the thermoswitch leads to a deviation of the activation temperature of the thermoswitch, therefore a deviation of the temperature of the roller. Sophisticated electronic circuitry with a temperature sensor can control the temperature much more accurately and consistently by continuous feedback process, and will not wear because there are no mechanical contacts in the circuit. But the cost of such control circuitry is very high, therefore these circuitries are employed only in some expensive models of the image forming apparatuses.
It is known that ceramic PTC (positive temperature coefficient of resistance) resistors can be used as electrical heating elements, such as in the applications disclosed in U.S. Pat. No. 4,899,032 to Schwarzl et al., for heating flooring media; and in U.S. Pat. No. 4,213,031 to Farber, for welding thermoplastic foils. A ceramic PTC resistor consists of polycrystalline ceramic materials having the characteristics of semiconductivity and ferroelectricity. The resistance of the materials does not change significantly with the increase of the temperature of the materials (hereafter referred to as the Temperature) until the Temperature reaches a characteristic value, which is often referred to as the Curie temperature. Above the Curie temperature, the resistance of the materials increases with the Temperature very dramatically, reaching 10.sup.2 to 10.sup.9 times higher than its value at room temperature over a narrow temperature range according to Kuwabara (in Advances in Ceramics, Vol. 7, American Ceramic Society, 1983, pages 128-145) and Holmes et al (in Advances in Ceramics, Vol. 7, American Ceramic Society, 1983, pages 146-155) in their published research works. FIG. 1 schematically shows a typical change of resistance of a ceramic PTC resistor with respect to the Temperature. The resistance, which is 10 ohm at room temperature, becomes 100,000 ohm above the Curie temperature, T.sub.c, corresponding to 10,000 times of the value at room temperature, which is still a moderate magnitude for this kind of PTC ceramic materials. This resistance change results in a change of heating power from 1440 watts to 0.144 watts with an electric power supply of constant 120 volt. With minimal heat dissipation, the Temperature will decrease quickly. When the Temperature decreases to lower than the operating temperature, there will be steep resistance decrease and heating power increase, resulting in a quick increase in temperature. This process automatically reaches an equilibrium state when the amount of heat dissipated is in balance with the amount of heat generated, corresponding to a resistance between 10 ohm and 100,000 ohm and a temperature between the Curie temperature, T.sub.c, and the run-away temperature, T.sub.r, that is, a point on the steeply rising slope of the curve in FIG. 1. PTC ceramic heating elements are usually designed in such a way that the heating rate at temperatures lower than Curie temperature is many times higher than the heat dissipation rate so that the Temperature can rise to and stay "locked in" at the Curie temperature a few seconds after the power-on. The Curie temperature of a PTC ceramic material can be adjusted by changing the composition of the material before the heating element is made and will remain unchanged during operation of the heating element. Resistor materials with Curie temperatures as high as 320.degree. C., as low as -50.degree. C., can be readily obtained this way by those skilled in the art according to Schwarzl et al. in their U.S. Pat. No. 4,899,032 and Hawkins in his U.S. Pat. No. 4,730,103.